BI0005 - Lecture 4 - nerves 4

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What is a general path for sensing and effecting?

A sensor (receptor) picks up information.

It sends a signal along an afferent pathway to the control center in the CNS.

This Control center then sends a signal along an efferent pathway to an effector cell.

What are Ligand Gated Ion channels?

They are ion channels capable of binding to the neurotransmitters. They are clustered in the membrane of the post-synaptic cell, directly opposite the synaptic cell.

What happens when neurotransmitters bind to the ligand gated ion channels?

Binding of the neurotransmitter to a particular part of the channel opens the channel and allows specific ions to diffuse across the postsynaptic membrane.

The result is generally a postsynaptic potential, a change in the membrane potential of the postsynaptic cell.

What are EPSPs?

They are excitatory postsynaptic potentials.

At certain synapses, the neurotransmitter binds to a type of channel through which both K+ and Na+ can diffuse.

When those channels open, the postsynaptic membrane depolarizes as the membrane potential approaches a value roughly midway between EK and ENa

Because these depolarization bring the membrane potential towards the threshold, they are called excitatory postsynaptic potentials.

What are IPSPs?

At other synapses, a differnt neurotransmitter binds to channels that are selectively permeable for only K+ or Cl-.

When those channels open, the postsynaptic membrane hyperpolarizes.

Hyperpolarizations produced in this manner are called inhibitory postsynaptic potentials because they move the membrane potential farther from the threshold.

What happens to neurotransmitters after they've had their effect in the synaptic cleft?

Various mechanisms rapidly clear neurotransmitters form the synaptic cleft, terminating their effect on postsynaptic cells.

Certain neurotransmitters may be actively transported into the pressynaptic neuron, to be repackaged into synaptic vesicles, or they may be transported into glia, to be metabolized as fuel.

Other neurotransmitters are removed from the synaptic cleft by simple diffusion of by an enzyme that catalyzes hydrolysis of the neurotransmitter.

How do postsynaptic potentials differ from action potentials?

Unlike action potentials, which are all-or-nothing events, postsynaptic potentials are graded; htie rmagnitude varies with a number of factors, including the amount of neurotransmitter released by the presynaptic neuron.

Furthermore, postsynaptic potentials usually do not regenerate as they spread along the membrane of a cell; they become smaller with distance from the synapse.

Most synapses on a neuron are located on its dendrites or cell body, whereas action potentials are generally initiated at the axon hillock. Therefore, a single EPSP is usually too small to trigger an action potential in a postsynaptic neuron.

What is temporal summation?

On some occasions, two EPSPs occur at a single synapse in such rapid succession that the post synaptic neuron's membrane potential has not returned to the resting potential before the arrival of the second EPSP. When that happens, the EPSPs add together, an effect called temporal summation.

What is spatial summation?

EPSPs produced nearly simultaneously by different synapses on the same postsynaptic neuron can also add together, and effect called spatial summation.

What can temporal and spatial summation achieve?

Through spatial and temporal summation several EPSPs can depolarize the membrane at the axon hillock to the threshold, causing the postsynaptic neuron to produce an action potential.

Summation applies as well to IPSPs: Two or more IPSPs occurring nearly simultaneously or in rapid succession have a larger hyperpolarizing effect than a single IPSP.

Through summation, an IPSP can also counter the effect of an EPSP.

What is convergence of input?

One cell is influenced by many others.

What is divergence of output?

One cell influences many others.

In the finger, what is a meissners corpuscle?

A rapidly adapting mechanoreceptor (touch and pressure)

In the finger, what is a merkles corpuscle?

A slowly adapting mechanoreceptor (touch and pressure)

In the finger what are free nerve endings?

Slowly adapting - some are nociceptors (pain), some are thermoreceptors, and some are mechanoreceptors.

In the finger, what are Pacinian corpuscles?

Rapidly adapting mechanoreceptors - vibration and deep pressure.

In the finger, what are ruffini corpuscles?

Slow adapting mechanoreceptor - skin stretch

What are the necessary characteristics of receptors?

High specificity

- Respond more readily to one particular stimulus

- Separate receptors for e.g. pressure, temperature.

High sensitivity

- Olfactory receptors can respond to a few odour molecules in the air.

- Visual receptors can respond to single photons.

Conversion of stimulus to action potential may require high degree of amplificaiton.

What is a reflex? Where is it produced? What is it for?

Sometimes the spinal cord acts independently of the brain as part of the simple nerve circuits that produce reflexes, the body's automatic responses to certain stimuli.

A reflex protects the body by triggering a rapid involuntary response to a particular stimulus.

For example, if you put your hand on a hot burner, a reflex begins to pull your hand back well before the sensation of pain has been processed in your brain.

During a physical exam, your doctor may trigger this knee-jerk reflex with a mallet to help assess nervous system function.

How does the knee-jerk reflex work?

1. The reflex is initiated artificially by tapping the tendon connected to the quadriceps muscle.

4. In response to signals from the sensory neurons, motor neurons convey signals to the quadriceps, causing it to contract and jerking the lower leg forward.

5. Sensory neurons also communicate with interneurons in the spinal cord.

6. The interneurons inhibit motor neurons that lead to the hamstring muscle. This inhibition prevents contraction of the hamstring which would resist the action of the quadriceps.

What are sensory receptors?

They are sensory cells and organs, as well as the structures within sensory cells that respond to specific stimuli.

What is sensory transduction and receptor potential?

The conversion of a physical or chemical stimulus to a change in the membrane potential of a sensory receptor is called sensory transduction, and the change in membrane potential itself is known as receptor potential.

Receptor potentials are graded potentials.

What is amplification?

Amplification refers to the strengthening of stimulus energy during transduction.

E.g. an action potential conducted from the eye to the human brain has about 100,000 times as much energy as the few photons of light that triggered it.

What is sensory adaptation?

Upon condinued stimulation, many receptors undergo a decrease in responsiveness termed sensory adaptation. Without sensory adaptation you would be constantly aware of feeling every beat of your heart and every bit of clothing on your body. Adaptation also enables you to see, hear, and smell changes in environments that very widely in stimulus intensity.